The only ham radio app I have used (so far) is EchoLink (although I swore I would NEVER use it!) and I have to say I am enjoying the heck out of it.
In the April 2010 edition of QST, Steve Ford has an article describing some iPod/iPad apps. One of them is iPSK31 that lets you send and receive PSK31 signals. There is also a followup article in the August 2010 edition that discusses how to make interface cables.
It looks like it is going to be a long cold winter, so I may try out some of these apps!
73,
de Scott NØIU